1. Field of the Invention
The present invention pertains to hydraulically-actuated and electrically-controlled downhole vibratory/impact source apparatus for generating seismic signals in conjunction with vertical seismic profiling-type logging or exploration processes.
2. Background
Certain well "logging" and hydrocarbon exploration processes are performed utilizing what are known as vertical seismic profiling or reverse vertical seismic profiling processes. In reverse vertical seismic profiling processes, a source of an impact signal is lowered into a wellbore to generate vibrations which are transmitted through the earth formation to an array of geophones located in either adjacent wellbores and/or on the earth's surface in a predetermined pattern. This process is useful in obtaining information about particular formation zones of interest surrounding the wellbore.
One constraint on providing suitable vertical seismic profiling processes is the limitations on providing suitable power sources downhole to generate repeated impact or vibratory signals to the earth formation which will provide suitable readings at the receiver elements or geophones. Since it is usually undesirable to damage the wellbore in which the source is disposed, explosive-type seismic sources are, in many instances, not usable. Moreover, heretofore, it has been impractical to transmit or generate enough electrical power "downhole" to provide adequate energy for a seismic source. However, with the development of combinations of tubing strings, notably coiled tubing, with electrical cable or so-called wireline-type electrical conductor arrangements for performing certain operations in a wellbore, it has been determined that it is possible to provide adequate hydraulic power through the tubing string to a downhole vibratory source as established by the embodiments of the present invention.